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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.21

Deferred Tool Search Experiment (tengu_tst_kx7)

An A/B test is running to evaluate enabling automatic tool search even when below the token threshold, if deferred tools are present in the context.

Details
  • Feature flag: tengu_tst_kx7 (controlled by GrowthBook)
  • Activates when: below normal tool search threshold BUT deferred tools exist
  • Hypothesis: Deferred tools signal potential tool use even at low token counts
  • Logs: "Tool search enabled via experiment (tengu_tst_kx7)" or "disabled via experiment"
  • Falls back silently if GrowthBook not ready: "tengu_tst_kx7: GrowthBook not ready, skipping"
Evidence

Line 451507 (experiment check, contains V4("tengu_tst_kx7", !1) and "via experiment (tengu_tst_kx7): below threshold, deferred tools present")

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.21 →